1. Field of the Invention
The present invention is generally in the field of night vision devices (NVD's) of the light-amplification type. Such NVD's employ an image intensifier tube (I.sup.2 T) to receive photons of light from a scene. This scene may be illuminated by full day light; or alternatively, the scene may be illuminated with light which is either of such a low level, or of such a long wavelength (i.e., infrared light), or both, that the scene is only dimly visible or is effectively invisible to the natural human vision. The I.sup.2 T responsively provides a visible image replicating the scene.
2. Related Technology
Even on a night which is too dark for natural human vision, invisible infrared light is richly provided in the near-infrared portion of the spectrum by the stars of the night sky. Human vision cannot utilize this infrared light from the stars because the infrared portion of the spectrum is invisible for humans. Under such conditions, a night vision device (NVD) of the light amplification type can provide a visible image replicating a night-time scene. Such NVD's generally include an objective lens which focuses invisible infrared light from the night-time scene through the transparent light-receiving face of an image intensifier tube (I.sup.2 T). At its opposite image-output face, the I.sup.2 T provides a visible image, generally in yellow-green phosphorescent light. This image is then presented via an eyepiece lens to a user of the device.
A contemporary NVD will generally use an I.sup.2 T with a photocathode (PC) behind the light-receiving face of the tube. The PC is responsive to photons of visible and infrared light to liberate photoelectrons. Because an image of a night-time scene is focused on the PC, photoelectrons are liberated from the PC in a pattern which replicates the scene. These photoelectrons arc moved by a prevailing electrostatic field to a microchannel plate having a great multitude of microchannels, each of which is effectively a dynode. These microchannels have an interior surface substantially defined by a material providing a high average emissivity of secondary electrons. In other words, each time an electron (whether a photoelectron or an electron previously emitted by the microchannel plate) collides with this material at the interior surface of the microchannels, more than one electron (i.e., secondary-emission electrons) leaves the site of the collision. This process of secondary electron emissions is not an absolute in each case, but is a statistical process having an average emissivity of greater than unity.
As a consequence, the photoelectrons entering the microchannels cause a geometric cascade of secondary-emission electrons moving along the microchannels, from one face to the other so that a spatial output pattern of electrons (which replicates the input pattern; but at a considerably higher electron density) issues from the microchannel plate.
This pattern of electrons is moved from the microchannel plate to a phosphorescent screen electrode by another electrostatic field. When the electron shower from the microchannel plate impacts on and is absorbed by the phosphorescent screen electrode, a visible image is produced. This visible image is passed out of the tube through a transparent image-output window for viewing.
The necessary electrostatic fields for operation of an I.sup.2 T are provided by an electronic power supply. Usually a battery provides the electrical power to operate this electronic power supply so that many of the conventional NVD's are portable.
However, the electrostatic fields maintained within a conventional image intensifier tube, and which are effective to move electrons from the photocathode to the screen electrode, also unavoidably move any positive ions which exist within the image intensifier tube toward the photocathode. Because such positive ions may include the nucleus of gas atoms of considerable size (i.e., of hydrogen, oxygen, and nitrogen, for example, all of which are much more massive than an electron), these positive gas ions are able to impact upon and cause physical and chemical damage to the photocathode.
Conventional image intensifier tubes have an unfortunately high indigenous population of such gas atoms within the tube--both those which become positive ions and those more populous atoms that become electrically neutral but possibly chemically active atoms within the tube. Historically, this indigenous population of gas atoms resulted in the impact of many positive ions on the photocathode, resulting in a relatively short operating life for many early-generation I.sup.2 T's.
As those ordinarily skilled in the pertinent arts will understand, later generation I.sup.2 T's of the proximity focus type have partially solved this ion-impact problem by providing an ion barrier film on the inlet side of the MCP. This ion barrier film blocks the positive ions and prevents them from damaging the PC. However, the ion barrier film is itself the source of many disadvantages.
A recognized disadvantage of such an ion barrier film on an MCP is the resulting decrease in signal to noise ratio provided by the MCP between a PC of an I.sup.2 T and the output screen electrode of the tube. That is, although the material of the ion barrier film acts as a secondary emitter of electrons for those electrons of sufficient energy, for lower energy photoelectrons this barrier also acts to preventing some of the electrons from reaching the microchannels of the MCP. Recalling that about 50% of the electron input face of a MCP is open area, and about the same percentage is defined by the solid portion or web of the microchannel plates, it is easily appreciated that about half of the photoelectrons impact on the web of the MCP.
These photoelectrons which impact the web of the plate bounce or rebound, or result in the production of secondary emission electrons, both conditions resulting in electrons closely adjacent to the face of the MCP with low energies. These low-energy electrons lack the energy to either penetrate the ion barrier film, or to cause this film to liberate secondary electrons. So these low energy electrons are absorbed by the ion barrier film. The result is that in some cases, as much as 50% of the electrons that would otherwise contribute to the formation of an image by the I.sup.2 T arc blocked or absorbed by the ion barrier film and do not reach the microchannels to be amplified as described above. Thus, about the same percentage of the image information which theoretically could be provided by the tube is lost.
Another disadvantage of the ion barrier film is that it contributes to halo effect in the image provided by the conventional image intensifier tube. This halo effect may be visualized as photoelectrons incident on the web of the MCP, or on the ion barrier film itself, either themselves not penetrating this film to enter a microchannel and to be amplified, but bouncing off to again impact the film or the web at another location. At the other location, the process is repeated, with some of the electrons entering a microchannel, and some of the electrons again bouncing to yet a third location. This effect causes a halo or emission of light around locations of the image that do not correspond to a bright area of the scene being viewed. This halo effect reduces the quality of the image provided by an image intensifier tube, and reduces contrast values in this image. Importantly, for those photoelectrons below a certain energy value, the ion barrier film itself acts as a gain block with respect to the halo effect.
Another problem with image intensifier tubes using an ion barrier film is the voltage that must be provided (i.e., by the use of a higher applied voltage between the PC and the I.sup.2 T) to photoelectrons simply to compensate for the energy barrier represented by the film itself. Efficient penetration of the ion barrier film by photoelectrons requires about 600 to 1000 volts of applied potential.
Yet another source of image halo in conventional MCP's results from the excessive distance maintained between the PC and the front face of the MCP in these conventional I.sup.2 T's. The conventional I.sup.2 T's generally have a gap from PC to MCP no less than about 250.mu. meter (+ or - about 5.mu. meter). It is recognized that an important factor in the extent or degree of halo effect is the spacing between the PC and the MCP of an I.sup.2 T. However, conventional I.sup.2 T's have not been able to provide a spacing as small at that achieved by the present invention.
Further, the conventional manufacturing processes for I.sup.2 T's are inadequate to remove a sufficient quantity of the indigenous gas molecules from MCP's. Thus, the use of ion barrier films to protect the PC's of conventional Gen III image intensifier tubes has become industry-standard practice. Were it not for the presence of the ion barrier film on conventional MCP's, the Gen III type of PC's would be destroyed by positive ion bombardment in as little as two hours of operation. That is, the present Gen III I.sup.2 T's would provide a service life of as little as two hours were they not provided with an ion barrier film on the MCP of these tubes. With the current ion barrier films, present Gen III I.sup.2 T's provide a service life of about 2000 hours or more.
U.S. Pat. No. 3,720,535, issued Mar. 13, 1973; U.S. Pat. No. 3,742,224, issued Jun. 26, 1973; and U.S. Pat. No. 3,777,201, issued Dec. 4, 1973 provide examples of microchannel plates or image intensifier tubes having an ion barrier film on a microchannel plate. U.S. Pat. Nos. 5,015,909; and 5,108,961, issued May 14, 1991, and Apr. 28, 1992, and both assigned to Circon Corporation provide examples of glass compositions and methods which may be used in the fabrication of microchannel plates.
U.S. Pat. No. 4,978,885, issued Dec. 18, 1990 asserts to provide a process by which a sufficient amount of the indigenous gas molecules may be removed from a MCP that it may be operated without an ion barrier film. However, the process taught by the '885 patent is believed to have several disadvantages. First of all, the MCP's treated by this process are exposed to an applied voltage sufficient to cause regenerative ion feedback, and are at or above the threshold of ion runaway. Further, the MCP's during this process are asserted to be self-heating so that it is asserted that no supplemental heating of the MCP is necessary (i.e., no externally applied heat for bake out under vacuum is required).
This combination of circumstances taught by the '885 patent is believed to subject the MCP's to a risk of damage or destruction because of thermal runaway that may accompany the condition of ion runaway. Further, even when a MCP is reversed and treated by the process of the '885 patent with the direction of self-regenerating ion feedback taking place near each of the opposite ends of the microchannels, there possibly can remain a central area of the microchannels where self-regenerating ion and electron flux is not sufficient to degas the MCP.